1. Field of the Invention
This invention relates to methods and systems for measuring neutron emissions and ionizing radiation, solid state detectors for use therein, and imaging systems and arrays of such detectors for use therein.
2. Background Art
The material requirements for a room temperature-operated, high resolution semiconductor gamma ray spectrometer include large free charge carrier mobilities (μ), or alternatively, high achievable free charge carrier velocities (ν), long mean free drift times (τ*), a relatively large energy band gap (Eg), generally between 1.4 eV to 2.5 eV, high representative values of atomic number (Z), and availability in large volumes. Presently, no semiconductor has all of the listed ideal material properties desired for the “perfect” room temperature-operated, semiconductor radiation spectrometer, although many have a considerable fraction, of the required properties. Some wide band gap compound semiconductors that offer promise as room temperature-operated, gamma ray spectrometers include HgI2, CdTe, and CdZnTe.
In addition to unique properties required for a room temperature-operated, gamma ray detector, HgI2, CdTe, and CdZnTe can also serve as neutron detectors. The isotope 113Cd, occurring in nature at a natural abundance of 12.3%, spontaneously emits gamma rays when it absorbs neutrons. Additionally, pure 113Cd has a thermal neutron cross-section of 20,000 barns, an enormously large value. Hence, CdTe and CdZnTe semiconductors can both absorb neutrons and then spontaneously emit the gamma ray reaction products. Natural Cd, due to the dilution with other Cd isotopes, has a cross-section of only 2450 barns. Further, CdTe and CdZnTe have reduced macroscopic (or linear) thermal neutron cross-section, after considering further dilution with Zn and Te atoms, of only 29.72/cm. Still, 29.72/cm is a reasonably large number, in that over 99% of impinging thermal neutrons will be absorbed within a thickness of 3 mm.
Similarly, 199Hg also absorbs neutrons and emits spontaneous gamma rays, and 199Hg occurs in nature at 16.9% natural abundance. Pure 199Hg has a thermal neutron microscopic cross-section of 2000 barns, and the semiconductor 199Hg has a macroscopic thermal neutron cross-section of 2.87/cm. A 3 mm thick HgI2 detector will absorb 57.7% of the thermal neutrons impinging upon it.
It has been shown that CdTe, CdZnTe, and HgI2 can be used to detect neutrons. In general, the 113Cd(n,γ)114Cd reaction has salient gamma ray emissions at 558.6 keV and 651.3 keV. It is these emissions that the CdTe or CdZnTe gamma ray spectrometers are to detect, and being gamma ray detectors, the system has been shown to work. The 199Hg(n,γ)200Hg reaction releases a single salient gamma ray emission at 368 keV which is easier to detect that the 113Cd(n,γ)114Cd emissions because of its lower energy. Again, the concept has been proven to work.
Unfortunately, a gamma ray field accompanies most neutron measurements. Since the described devices are gamma ray detectors, confusion can arise regarding the difference between neutron-induced gamma rays and background gamma rays. Even if the energy resolution of the detectors is superb, the Compton scattering continuum remains a major portion of the established counts in the detection spectrum, and feature that will blend all Compton scattering gamma ray counts in the spectrum. Hence, a method is needed to confidently discriminate between neutron-induced gamma ray counts and background gamma ray counts.
A method is described in the literature that allows for gamma ray discrimination, yet it has problems. In the literature, Beyerle and Hull describe using HgI2 with the common natural abundance of 199Hg. They first shield gamma rays with at least 2.5 inches of lead between a neutron source and the detector. The measurement was taken. Afterward, a second spectrum was taken with an additional 0.0625 inch thick sheet of Cd placed between the neutron source and the detector. The Cd sheet effectively removes the neutrons. The second spectrum was then subtracted from the first to reveal the neutron-induced spectrum. The method demonstrated that HgI2 with natural Hg could be used for a detector, however the accuracy of the measurement is questionable. Firstly, it has been demonstrated that lead removes neutrons through scattering and absorption when placed between the neutron source and the detector. Hence, using lead as a shield reduces the device sensitivity to neutrons. Secondly, natural Cd (which has 113Cd in it) generates gamma rays, which become part of the background. Hence, placing Cd in the beam increases the gamma ray background higher than measured without the Cd. As a result, the actual gamma ray background is different for both devices and the subtracted spectrum produces an erroneous lower number of a neutron count. Then, Cd sheet can be placed far away from the device (2.5 meters in the literature) to reduce the gamma ray background generated by the 113Cd(n,γ)114Cd reaction, yet the arrangement does not allow for the realization of a compact detecting instrument.
U.S. Pat. No. 6,175,120 to McGregor et al. discloses a high-resolution, solid state, ionization detector and an array of such detectors.
The following U.S. Pat. Nos. are also relevant: 6,252,923; 5,969,359; 5,940,460; 5,726,453; 5,659,177; 5,083,028; 4,851,687; and 4,757,202.